CROSS-REFERENCE TO RELATED APPLICATION
BACKGROUND
1. Field of the Disclosure
[0002] The disclosure relates to a method of manufacturing a solar cell, and more particularly,
a method of manufacturing a solar cell including a dopant layer.
2. Description of the Related Art
[0003] Recently, as existing energy resources such as oil or coal are expected to be exhausted,
an interest in alternative energy for replacing oil or coal is increasing. In particular,
a solar cell that directly converts or transforms solar energy into electricity using
a semiconductor member is gaining attention.
A solar cell is manufactured by forming a plurality of layers and electrodes according
to a design. However, due to the plurality of layers and electrodes, manufacturing
cost of the solar cell increases and productivity of the solar cell decreases. Particularly,
[0004] in a solar cell having a p-n junction formed by a dopant layer, the cost is increased
by a process forming the dopant layer. Therefore, a technique being able to enhance
properties of the solar cell and to reduce the manufacturing cost of the solar cell
is needed.
[0005] EP 2 105 972 A2 discloses a method for manufacturing a photoelectric conversion device and that the
introduction of an impurity element imparting one conductivity type may be performed
in a manner similar to the formation of the first impurity semiconductor layer and
can be performed by an ion doping method, an ion implantation method or a laser doping
method.
[0007] US 2009/0117680 A1 discloses forming impurity semiconductor layers by doping with B
2H
6 and BF
3; P or As, respectively.
SUMMARY
[0008] Embodiments of the invention are directed to a a method of manufacturing a solar
cell being able to enhance properties of the solar cell and to reduce a manufacturing
cost of the solar cell.
A method of manufacturing a solar cell according to the invention includes the steps
of: forming an emitter layer by ion-implanting a first conductive type dopant to a
first surface of a semiconductor substrate; forming a back surface field layer by
ion-implanting a second conductive type dopant to a second surface of the semiconductor
substrate, doping with an additional dopant, wherein the additional dopant is a dopant
other than the first and second conductive type dopants, an amount of the additional
dopant doped during the forming the back surface field layer is larger than an amount
of the additional dopant doped during the forming the emitter layer, and the additional
dopant includes hydrogen, wherein the method is characterized by that
the first conductive type dopant is ion-implanted by an ion-selection using a mass
spectrometer in the forming the emitter layer, and the second conductive type dopant
is ion-implanted without the ion-selection using the mass spectrometer in the forming
the back surface field layer.
[0009] A solar cell according to an example useful for understanding the invention includes
a semiconductor substrate having a first surface and a second surface opposite to
each other; an emitter layer formed at the first surface of the semiconductor substrate,
wherein the emitter layer includes a first conductive type dopant; and a back surface
field layer formed at the second surface of the semiconductor substrate, wherein the
back surface field layer includes a second conductive type dopant. When an additional
dopant is a dopant other than the first and second conductive type dopants, the concentration
of the additional dopant in the back surface field layer is higher than the concentration
of the additional dopant in the emitter layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
FIG. 1 is a cross-sectional view of a solar cell according to an example useful for
understanding the invention.
FIG. 2 is a block diagram illustrating a method of manufacturing a solar cell according
to an embodiment of the invention.
FIGs. 3a to 3f are cross-sectional views illustrating the method of manufacturing
the solar cell according to an embodiment of the invention.
FIG. 4 is a schematic diagram illustrating an example of an ion-implantation apparatus
used in a step of forming an emitter layer in the method of manufacturing the solar
cell according to an embodiment of the invention.
FIG. 5 is a cross-sectional view of a solar cell according to another example useful
for understanding of the invention.
FIG. 6 is a graph of phosphorous concentration and hydrogen concentration with respect
to a depth from a surface of a semiconductor substrate of a solar cell manufactured
by Experimental Embodiment.
FIG. 7 is a graph of phosphorous concentration and hydrogen concentration with respect
to a depth from a surface of a semiconductor substrate of a solar cell manufactured
by Comparative Example.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0011] Hereinafter, embodiments of the invention will be described with reference to the
accompanying drawings.
[0012] In order to clearly and concisely illustrate the embodiments of the invention, members
not related to the embodiments of the invention are omitted in the figures. Also,
members similar to or the same as each other may have the same reference numerals.
In addition, the dimensions of layers and regions may be exaggerated or schematically
illustrated, or some layers may be omitted for clarity of illustration. In addition,
the dimension of each part as drawn may not reflect an actual size.
[0013] In the following description, when a layer or substrate "includes" another layer
or portion, it can be understood that the layer or substrate further includes still
another layer or portion. Also, when a layer or film is referred to as being "on"
another layer or substrate, it can be understood that the layer of film is directly
on the other layer or substrate, or intervening layers are also present. Further,
when a layer or film is referred to as being "directly on" another layer or substrate,
it can be understood that the layer or film is directly on the another layer or substrate,
and thus, there is no intervening layer.
[0014] Hereinafter, a solar cell and a method of manufacturing the same according to embodiments
of the invention will be described with reference to the accompanying drawings.
[0015] FIG. 1 is a cross-sectional view of a solar cell according to an example useful for
understanding the invention.
[0016] Referring to FIG. 1, a solar cell 100 according to an example useful for understanding
the invention includes a semiconductor substrate 10, an emitter layer 20 formed at
or adjacent to a first surface (hereinafter, referred to as "a front surface") of
the semiconductor substrate 10, a back surface field layer 30 formed at or adjacent
to a second surface (hereinafter, referred to as "a back surface") of the semiconductor
substrate 10, an anti-reflection film 22 and a first electrode (or a plurality of
first electrodes) 24 formed on the front surface of the semiconductor substrate 10,
and a passivation film 32 and a second electrode(or a plurality of second electrodes)
34 formed on the back surface of the semiconductor substrate 10. This will be described
in more detail.
[0017] The semiconductor substrate 10 may include one or more of various semiconductor materials.
For example, the semiconductor substrate 10 includes silicon having a dopant of a
second conductivity type. Single crystal silicon or polycrystalline silicon may be
used for the silicon, and the second conductivity type may be an n-type. That is,
the semiconductor substrate 10 may include single crystal silicon or polycrystalline
silicon having a group V element, such as phosphorus (P), arsenic (As), bismuth (Bi),
antimony (Sb), or the like.
[0018] When the semiconductor substrate 10 has the n-type dopant as in the above, the emitter
layer 20 of a p-type is formed at the front surface of the semiconductor substrate
10, and thereby forming a p-n junction. When the sunlight is incident to the p-n junction,
the electrons generated by the photoelectric effect moves to the back surface of the
semiconductor substrate 10 and are collected by the second electrode 34, and the holes
generated by the photoelectric effect moves to the front surface of the semiconductor
substrate 10 and are collected by the first electrode 24. Thus, the electric energy
is generated.
[0019] Here, the holes having mobility lower than that of the electrons move to the front
surface of the semiconductor substrate 10, and not to the back surface of the semiconductor
substrate 10. Therefore, the conversion efficiency of the solar cell 100 can be enhanced.
[0020] The front and back surfaces of the semiconductor substrate 10 may be a textured surface
to have protruded and/or depressed portions of various shapes (such as pyramid shape).
Thus, the surface roughness is increased by the protruded and/or depressed portions,
and reflectance of the incident sunlight at the front surface of the semiconductor
substrate 10 can be reduced by the texturing. Then, an amount of the light reaching
the p-n junction between the semiconductor substrate 10 and the emitter layer 20 can
increase, thereby reducing an optical loss of the solar cell 100.
[0021] However, the invention is not limited thereto, and thus, the protruded and/or depressed
portions may be formed at only one surface (especially, the front surface), or there
may be no protruded and/or depressed portions at the front and back surfaces.
[0022] The emitter layer 20 of the first conductive type may be formed at the front surface
of the semiconductor substrate 10. A p-type dopant such as a group III element (for
example, boron (B), aluminum (Al), gallium (Ga), indium (In) or the like) may be used
for the first dopant 202. In the embodiment, the emitter layer 20 may be formed by
an ion-implantation method, and this will be described later in more detail.
[0023] For example, the emitter layer 20 has a dopant surface concentration of about 1.0X10
18∼1X10
20 atoms/cm
3, and has a thickness about 0.3∼1.8
µm. The above surface concentration and the thickness are determined with consideration
of the photoelectric conversion efficiency and the contact property with the first
electrode 24. However, the invention is not limited thereto. Thus, the surface concentration
and the thickness may be varied.
[0024] The anti-reflection film 22 and the first electrode 24 may be formed on the emitter
20 at the front surface of the semiconductor substrate 10.
[0025] The anti-reflection film 22 may be substantially or entirely formed at the front
surface of the semiconductor substrate 10, except for the portion where the first
electrode 24 is formed. The anti-reflection film 22 reduces reflectance (or reflectivity)
of sunlight incident to the front surface of the semiconductor substrate 10. Also,
the anti-reflection film 22 passivates defects at a surface or a bulk of the emitter
layer 20.
[0026] By reducing the reflectance of sunlight incident to the front surface of the semiconductor
substrate 10, an amount of the sunlight reaching the p-n junction formed between the
semiconductor substrate 10 and the emitter layer 20 can be increased, thereby increasing
short circuit current (Isc) of the solar cell 100. Also, by passivating the defects
at the emitter layer 20, recombination sites of minority carrier are reduced or eliminated,
thereby increasing an open-circuit voltage (Voc) of the solar cell 100. Accordingly,
the open-circuit voltage and the short-circuit current of the solar cell 100 can be
increased by the anti-reflection film 22, and thus, the efficiency of the solar cell
100 can be enhanced.
[0027] The anti-reflection film 22 may include one or more of various materials. For example,
the anti-reflection film 22 may include a silicon nitride layer. However, the invention
is not limited thereto. Thus, the anti-reflection film 22 may have a single film structure
or a multi-layer film structure including, for example, at least one material selected
from a group including silicon nitride, silicon nitride including hydrogen, silicon
oxide, silicon oxy nitride, MgF
2, ZnS, TiO
2, and CeO
2. Here, when the anti-reflection film 22 includes Al
2O
3 layer formed on (for example, formed directly on) the emitter layer 20 and a silicon
nitride layer formed on (for example, formed directly on) the Al
2O
3 layer, the passivation property of the emitter layer 20 of the p-type can be maximized.
However, the invention is not limited thereto. The anti-reflection film 22 may include
one or more of various materials.
[0028] The first electrode 24 is electrically connected to the emitter layer 20 by penetrating
the anti-reflection film 22 at the front surface of the semiconductor substrate 10.
The first electrode 24 may include one or more of various metals having high electrical
conductivity. For example, the first electrode 24 may include silver (Ag) having high
electrical conductivity. However, the invention is not limited thereto. The first
electrode 24 may be a single layer including transparent conductive materials, or
may have a stacked structure having a transparent conductive layer and a metal layer
(called "a bus bar" or "a finger electrode") on the transparent conductive layer.
[0029] Also, the back surface field layer 30 including the second conductive type dopant
is formed at the back surface of the semiconductor substrate 10. The conductive type
dopant of the back surface field layer 30 may be the same as or different from the
conductive type dopant of the semiconductor substrate 10 where the emitter layer 20
and the back surface field layer 30 are not formed.
[0030] The back surface field layer 30 can minimize the recombination of the electrons and
the holes, and enhance the efficiency of the solar cell 100. The back surface field
layer 30 may include phosphorus (P), arsenic (As), bismuth (Bi), antimony (Sb), or
the like as the second conductive type dopant. In the embodiment, the back surface
field layer 30 may be formed by an ion-implantation method, and this will be described
later in more detail.
[0031] For example, the surface concentration of the second conductive type dopant in the
back surface field layer 30 is in a range of about 1.0X10
18∼5X10
21 atoms/cm
3, and a thickness of the back surface field layer 30 is about 0.3∼1.8
µm. The above surface concentration and the thickness are determined with consideration
of the back surface field effect and the contact property with the second electrode
34. The surface concentration of the second conductive type dopant in the back surface
field layer 30 may be higher than the surface concentration of the first conductive
type dopant in the emitter layer 20. However, the invention is not limited thereto.
Thus, the surface concentration and the thickness of the back surface field layer
30 may be varied.
[0032] The passivation film 32 and the second electrode 34 may be formed at the back surface
of the semiconductor substrate 10.
[0033] The passivation film 32 may be substantially or entirely formed at the back surface
of the semiconductor substrate 10, except for the portions where the second electrode
34 is formed. The passivation film 32 passivates defects at the back surface of the
semiconductor substrate 10, and eliminates the recombination sites of minority carrier.
Thus, an open circuit voltage of the solar cell 100 can be increased.
[0034] The passivation film 32 may include a transparent insulating material for passing
the light. Thus, the light can be incident to the back surface of the semiconductor
substrate 10 through the passivation film 32, and thereby enhancing the efficiency
of the solar cell 100. The passivation film 32 may have a single film structure or
a multi-layer film structure including, for example, at least one material selected
from a group including silicon nitride, silicon nitride including hydrogen, silicon
oxide, silicon oxy nitride, MgF
2, ZnS, TiO
2, and CeO
2. However, the invention is not limited thereto, and thus, the passivation film 32
may include one or more of various materials.
[0035] The second electrode 34 may include a metal having a high electric conductivity.
For example, the second electrode 34 may include silver (Ag) having high electrical
conductivity and high reflectance. When the second electrode 34 includes the silver
having high reflectance, the second electrode 34 can reflect the light toward the
back surface of the semiconductor substrate 10 and the light is reflected toward the
inside of the semiconductor substrate 10. Thus, the amount of the light captured can
be increased.
[0036] The second electrode 34 may have a width larger than that of the first electrode
24, and a number of the second electrode 34 may be greater than that of the first
electrode 24. Accordingly, the total area of the second electrode 34 may be larger
than that of the first electrode 24. However, the invention is not limited thereto.
[0037] In the embodiment, the semiconductor substrate 10 is the n-type, and the emitter
layer 20 is the p-type. However, the invention is not limited thereto. Therefore,
the semiconductor substrate 10 may be the p-type, and the emitter layer 20 may be
the n-type. That is, various modifications are possible.
[0038] As mentioned above, in the embodiment, each of the emitter layer 20 and the back
surface field layer 30 is formed by the ion-implantation method. Here, the emitter
layer 20 and the back surface field layer 30 are formed under different conditions.
Especially, the concentrations of an additional dopant other than the first and second
conductive type dopants in the emitter layer 20 and the back surface field layer 30
are different. More specifically, the first conductive type dopant is ion-implanted
by an ion-selection using a mass spectrometer when the emitter layer 20 is formed.
On the other hand, the second conductive type dopant is ion-implanted without the
ion-selection using the mass spectrometer when the back surface field layer 30 is
formed. Accordingly, the concentration of the additional dopant in the back surface
field layer 30 is higher than the concentration of the additional dopant in the emitter
layer 20. This will be described later in more detail.
[0039] The additional dopant may include hydrogen. For example, in at least a region of
the back surface field layer, the hydrogen concentration may be higher than the concentration
of the second conductive type dopant. More particularly, a portion of the back surface
field layer 30 apart from the back surface of the semiconductor substrate 10 by about
0.1∼0.4
µm may include an region having the hydrogen concentration (for example, in a range
of about 1X10
18∼1X10
20atoms/cm
3) higher than the concentration of the second conductive type dopant. The hydrogen
concentration of the back surface field layer 30 is determined in order to reduce
the contact resistance with the second electrode 34 while maintaining the other properties.
However, the invention is not limited thereto. On the other hand, in a portion of
the emitter layer 20 apart from the front surface of the semiconductor substrate 10
by about 0.1∼0.4
µm, the hydrogen concentration may be smaller than the first conductive type dopant,
and the hydrogen concentration of the portion may be in a range of about 1X10
15∼9X10
17atoms/cm
3. The hydrogen concentration of the emitter layer 20 is determined so that various
properties can be maintained. However, the invention is not limited thereto. Here,
the surface of the semiconductor substrate 10 is a surface of the semiconductor substrate
10 including the emitter layer 20 or the back surface field layer 30.
[0040] In the embodiment, the concentration of the additional dopant in the back surface
field layer 30 is higher than the concentration of the additional dopant in the emitter
layer 20. Thus, the contact resistance with the second electrode 34 can be reduced
by the additional dopant of the back surface field layer 30, while maintaining the
other properties of the solar cell 100.
[0041] In more detail, when the additional dopant other than the first conductive type dopant
is included in the emitter layer 20, generated carriers are captured by defects and
the current by short wavelength may decrease. That is, when the amount of the additional
dopant in the emitter layer 20 increases, the properties of the solar cell 100 may
be deteriorated. Accordingly, in the embodiment, when the first conductive type dopant
is ion-implanted in order to form the emitter layer 20, the amount of the additional
dopant is reduced (preferably, the additional dopant is not included in the emitter
layer 20). Therefore, the properties of the solar cell 100 can be maintained.
[0042] On the other hand, the surface concentration of the second conductive type dopant
in the back surface field layer 30 is higher than the surface concentration of the
first conductive type dopant in the emitter layer 20. Therefore, the additional dopant
rarely influences to the properties of the solar cell 100. Also, the area of the second
electrode 34 is larger than that of the first electrode 24. That is, the contact resistance
with the second electrode 34 having a relatively large area can be reduced. Accordingly,
fill factor of the solar cell 100 can be enhanced.
[0043] Also, during the ion-implanting for forming the back surface field layer 30, the
mass spectrometer is not used and the manufacturing cost can be effectively reduced.
[0044] Hereinafter, a method for forming a solar cell 100 (particularly, the steps of forming
an emitter layer 20 and a back surface field layer 30) will be described in more detail.
In the following description, the described portions in the above will be omitted,
and the not-described portions in the above will be described in detail.
[0045] FIG. 2 is a block diagram for illustrating a method of manufacturing a solar cell
according to an embodiment of the invention.
[0046] Referring to FIG. 2, a method of manufacturing a solar cell according to the embodiment
includes a step ST10 for preparing a semiconductor substrate, a step ST20 for forming
an emitter layer, a step ST30 for forming a back surface field layer, a step ST40
for heat-treating, a step ST50 for forming an anti-reflection film and a passivation
film, and a step ST60 for forming an electrode.
[0047] First, as shown in FIG. 3a, in the step ST10 for preparing the semiconductor substrate,
a semiconductor substrate 10 having a second conductive type dopant is prepared. The
front and back surfaces of the silicon semiconductor substrate 10 may be textured
to have protruded and/or dented portions of various shapes (or to have an uneven surface).
For the texturing method, a wet etching method or a dry etching method may be used.
In the wet etching method, the substrate 10 may be dipped into a texturing solution.
According to the wet etching method, the process time can be short. In the dry etching
method, the surface of the semiconductor substrate 10 is etched by a diamond drill
or a laser. In the dry etching, the protruded and/or dented portions can be uniformly
formed; however, the semiconductor substrate 10 may be damaged and the process time
may be long. Accordingly, the semiconductor substrate 10 may be textured by one or
more of various methods.
[0048] Next, as shown in FIG. 3b, in the step ST20 for forming the emitter layer, an emitter
layer 20 is formed at the front surface of the semiconductor substrate 10 by ion-implanting
a first conductive type dopant. More specifically, as shown in FIG. 4, the first conductive
type dopant is ion-implanted by an ion-selection using a mass spectrometer 220.
[0049] FIG. 4 is a schematic diagram for illustrating an example of an ion-implantation
apparatus used in a step of forming the emitter layer in the method of manufacturing
the solar cell according to an embodiment of the invention. The elements not directly
related to the embodiment (for example, a deflector, an accelerator, and so on) will
not shown in FIG. 4 and not be further described. However, the invention is not limited
thereto. Thus, various elements may be applied to the ion-implantation apparatus.
[0050] The ion-implantation apparatus 200 according to the example includes an ion source
210 supplying ion corresponding to the ion-implanted dopant, a mass spectrometer 220
for performing an ion-selection by a mass spectrometry of an ion beam 212 supplied
by the ion source 210, and an ion-implantation chamber 230 where the ion-implantation
is performed.
[0051] The ion source 210 may be an ion supplier of known structure and may be driven by
one of the known methods.
[0052] The mass spectrometer 220 may include a mass spectrometry magnet 222 providing magnetic
field to the ion beam 212 and a mass spectrometry slit 224 having passages where only
the predetermined ion passes through.
[0053] A semiconductor substrate 10 is fixed to a holder 232 in the ion-implantation chamber
230 in order for the ion passing through the mass spectrometry slit 224 to implant
to the semiconductor substrate 10. The holder 232 may move up and down and/or left
and right, and one of known structures may be applied for the movement.
[0054] A mass spectrometry of the ion beam 212 supplied from the ion source 210 is performed
by the mass spectrometry magnet 222 and the mass spectrometry slit 224.
[0055] That is, the first conductive type dopant, which is used for the ion-implantation
among the ion beam 212, travels the path that is formed by the mass spectrometry magnet
222 (refer to a dotted line of FIG. 4) and able to pass through the mass spectrometry
slit 224. Here, for high purity, the first conductive type dopant having atomic weight
of a predetermined range pass through the mass spectrometry slit 224 only. Also, the
ion, which is not used for the ion-implantation among the ion beam 212, travels the
path that is formed by the mass spectrometry magnet 222 (refer to an alternated long
and short dash line of FIG. 4) and not able to pass through the mass spectrometry
slit 224. Hereby, the first conductive type dopant, which is used for the ion-implantation
among the ion beam 212, is ion-selected and passes through the mass spectrometry slit
224.
[0056] The ion passing through the mass spectrometry slit 224 is implanted to the semiconductor
substrate 10, thereby forming the emitter layer 20 at the semiconductor substrate
10, as shown in FIG. 3b. For example, the dose of the hydrogen may be about 5X10
13atoms/cm
2 or less.
[0057] Next, as shown FIG. 3c, in the step ST30 for forming the back surface field layer,
a back surface field layer 30 is formed at the back surface of the semiconductor substrate
10 by ion-implanting a second conductive type dopant. Here, the second conductive
type dopant for forming the back surface field layer 30 is ion-implanted without the
ion-selection that uses the mass spectrometer.
[0058] Accordingly, an amount of the additional dopant in the back surface field layer 30
is larger than an amount of the additional dopant in emitter layer 20. For example,
the additional dopant may include hydrogen, and the dose of the hydrogen may be about
1X10
14∼5X10
15 atoms/cm
2 in the step ST30 for forming the back surface field layer. The additional dopant
may further include at least one additional material selected from the group including
potassium (K), calcium (Ca), chrome (Cr), manganese (Mn), iron (Fe), nickel (Ni),
and copper (Cu).
[0059] As in the above, in the ion-implantation for the back surface field layer 30 that
does not directly influence the properties of the solar cell 100, the ion-selection
using the mass spectrometer is not performed. Accordingly, the contact resistance
with the second electrode 34 can be reduced while maintaining the other properties
of the solar cell 100. Also, the mass spectrometer is not used, and thus, the manufacturing
cost can be reduced.
[0060] In the above, the back surface field layer 30 is formed after the emitter layer 20
is formed. However, the invention is not limited thereto. Thus, the emitter layer
20 can be formed after the back surface field layer 30 is formed.
[0061] Next, in the step ST40 for heat-treating, the first conductive type dopant ion-implanted
for forming the emitter layer 20 and the second conductive type dopant ion-implanted
for forming the back surface field layer 30 are activated by annealing the semiconductor
substrate 10.
[0062] When the first and second conductive type dopants are ion-implanted to the semiconductor
substrate 10, the first and second conductive type dopants are not activated since
the dopant are not positioned at the lattice sites. By annealing the semiconductor
substrate 10, the first and second conductive type dopants of the emitter layer 20
and the back surface field layer 30 move to the lattice sites and are activated. Also,
the first and second conductive type dopants diffuse into the inside of the semiconductor
substrate 10.
[0063] The heat-treating processes for activating the first and second conductive type dopants
may be performed simultaneously or sequentially. Here, when the heat-treating process
for activating the first and second conductive type dopants is performed simultaneously,
the process can be simplified.
[0064] The temperature and the other conditions may be varied according to materials of
the first and second conductive type dopants and the semiconductor substrate 10. For
example, the temperature may be about 950∼1300°C. However, the invention is not limited
thereto.
[0065] The hydrogen concentration in the back surface field layer 30 after the heat-treating
at step ST40 may decrease, compared with before the heat treating at step ST40. Accordingly,
the solar cell 100 is not deteriorated. In this case, the hydrogen concentration in
the back surface field layer 30 is higher than hydrogen concentration in the emitter
layer 20.
[0066] For example, after the heat-treating at step ST40, a portion of the back surface
field layer 30 apart from the back surface of the semiconductor substrate 10 by about
0.1∼0.4
µm may include a region having the hydrogen concentration in a range of about 1X10
18∼1X10
20 atoms/cm
3. On the other hand, after the heat-treating at step ST40, a portion of the emitter
layer 20 apart from the front surface of the semiconductor substrate 10 by about 0.1∼0.4
µm may include a region having the hydrogen concentration in a range of about 1X10
15∼9X10
17 atoms/cm
3.
[0067] However, the invention is not limited thereto. After the heat-treating at step ST40,
the hydrogen concentration in the back surface field layer 30 may fall to a level
similar to the hydrogen concentration in the emitter layer 20.
[0068] Also, after the heat-treating at step ST40, the additional material such as potassium,
calcium, chrome, manganese, iron, nickel, and copper may be included to a ND (not-detected)
level that is regarded as "not included". That is, the additional material such as
potassium, calcium, chrome, manganese, iron, nickel, and copper is not detected. Thus,
the other properties of the solar cell 100 can be maintained.
[0069] Next, as shown in FIG. 3e, in the step ST50 for forming the anti-reflection film
and the passivation film, the anti-reflection film 22 and the passivation film 32
are formed on the front surface and the back surface of the semiconductor substrate
10, respectively.
[0070] The anti-reflection film 22 and the passivation film 32 may be formed by one or more
of various methods such as a vacuum evaporation, a chemical vapor deposition, a spin
coating, a screen printing, or a spray coating.
[0071] Next, as shown in FIG. 3f, in the step ST60 for forming the electrode, a first electrode
24 electrically connected to the emitter layer 20 is formed at the front surface of
the semiconductor substrate 10 and a second electrode 34 electrically connected to
the back surface field layer 30 (or, the semiconductor substrate 10) is formed at
the back surface of the semiconductor substrate 10.
[0072] After forming an opening at the anti-reflection film 22, the first electrode 24 may
be formed inside the opening by one or more of various methods, such as a plating
method or a deposition method. Also, after forming an opening at the passivation film
32, the second electrode 34 may be formed inside the opening by one or more of various
methods, such as a plating method or a deposition method.
[0073] Selectively, the first and second electrodes 24 and 34 may be formed by fire-through
or laser firing contact of printed pastes for the first and second electrodes 24 and
34. For example, the pastes may be printed by one or more of various methods such
as a screen printing method. In this case, because the openings are naturally formed
during the fire-through or the laser firing contact, the steps for forming the openings
are not necessary.
[0074] In the above, each of the emitter layer 20 and the back surface field layer 30 has
uniform doping concentration. However, the invention is not limited thereto. At least
one of the emitter layer 20 and the back surface field layer 30 may have a selective
structure.
[0075] That is, as shown in FIG. 5, the emitter layer 20 includes a first portion 20a formed
adjacent to the anti-reflection film 22 between the front electrode 24, and a second
portion 20b being in contact with the front electrode 24. The second portion 20b has
a doping concentration higher than that of the first portion 20a, and thus, the second
portion 20b has a resistance lower than that of the first portion 20a.
[0076] Then, a shallow emitter can be achieved at the first portion 20a where the sunlight
is incident, and thereby enhancing the efficiency of the solar cell 100. In addition,
contact resistance with the front electrode 24 can be reduced at the second portion
20b being in contact with the front electrode 24. That is, when the emitter layer
20 has the selective emitter structure, the efficiency of the solar cell 100 can be
maximized.
[0077] Also, the back surface field layer 30 includes a first portion 30a formed at a portion
corresponding to a portion between the back electrode 34, and a second portion 30b
being in contact with the back electrode 34. The second portion 30b has a doping concentration
higher than that of the first portion 30a, and thus, the second portion 30b has a
resistance lower than that of the first portion 30a.
[0078] Then, the first portion 30a of the back surface field layer 30 effectively prevents
the recombination of the electrons and the holes, and the contact resistance with
the back electrode 34 can be reduced by the second portion 30b having a relatively
low resistance. Therefore, the loss by the recombination of the electrons and the
holes is reduced, and the electrons or the holes generated by the photoelectric effect
can be effectively transferred to the back electrode 34. Accordingly, the efficiency
of the solar cell 100 can be improved more.
[0079] The emitter layer 20 and the back surface field layer 30 having the selective structures
may be formed by one or more of various methods. For example, a comb mask may be used
so that the second portions 20b and 30b have a relatively high doping concentration
and the first portions 20a and 30a have a relatively low doping concentration. Selectively,
the emitter layer 20 and the back surface field layer 30 having the selective structures
may be formed by additionally doping the dopant only to the second portions 20b and
30b.
[0080] Hereinafter, embodiments of the invention will be described in more detail through
an experimental example. The experimental example is provided only for illustrative
purpose of the embodiments of the invention and the embodiments of the invention are
not limited thereto.
Experimental Embodiment
[0081] An n-type semiconductor substrate was prepared. Boron as a p-type dopant was ion-implanted
to a front surface of the semiconductor substrate to form an emitter layer, and phosphorus
as a n-type dopant was ion-implanted to a back surface of the semiconductor substrate
to form a back surface field layer. A co-activation was performed by annealing the
semiconductor substrate at 1000°C. Here, an ion-selection using a mass spectrometer
was performed during the ion-implantation forming the emitter layer; however, the
ion-selection using the mass spectrometer was not performed during the ion-implantation
forming the back surface field layer.
[0082] An anti-reflection film was formed on the front surface of the semiconductor substrate,
and a passivation film was formed on the back surface of the semiconductor substrate.
Then, a first electrode electrically connected to the emitter layer and a second electrode
electrically connected to the back surface field layer were formed, and a solar cell
was manufactured.
Comparative Example
[0083] A solar cell was manufactured by the same method in Experimental Embodiment except
that an ion-selection was performed in the ion-implantation for forming the back surface
field layer.
[0084] A dose of the additional dopant (hydrogen) during the ion-implantation for forming
the back surface field layer in Experimental Embodiment and Comparative Example are
shown in Table 1. Phosphorous concentration and hydrogen concentration with respect
to a depth from a surface of a semiconductor substrate of a solar cell manufactured
by Experimental Embodiment and Comparative Example are shown in FIG. 6 and 7, respectively.
Also, the additional materials were detected by a total reflection X-ray fluorescence
spectrometer after the heat-treating in Experimental Embodiment, and the result is
shown in Table 2.
[Table 1]
|
Experimental Embodiment |
Comparative Example |
Dose of an additional dopant |
1X1015 atoms/cm2 |
5X1013 atoms/cm2 |
[Table 2]
|
Potassium |
calcium |
chrome |
manganese |
iron |
nickel |
copper |
Detection level |
ND(Not detected) |
ND |
ND |
ND |
ND |
ND |
ND |
[0085] Referring to Table 1, the dose of the additional dopant in Experimental Embodiment
is larger than that in Comparative Example.
[0086] Referring to FIG. 6, it can be seen that a portion of the back surface field layer
apart from the surface of the semiconductor substrate by about 0.1∼0.4
µm may include an region having the hydrogen concentration in a range of about 1X10
18∼1X10
20 atoms/cm
3. In the hydrogen concentration range, the contact resistance with the second electrode
can be reduced and the other properties of the back surface field layer can be maintained.
On the other hand, in Comparative Example, the hydrogen concentration in the back
surface field layer is in a range about 1X10
17 atoms/cm
3. That is, it can be seen that the hydrogen concentration in the back surface field
layer in Comparative Example is low.
[0087] Also, referring to Table 2, an additional material such as potassium, calcium, and
so on is not detected at the back surface field layer in Experimental Embodiment.
That is, the solar cell according to Experimental Embodiment does not deteriorate
and the other properties are maintained.
[0088] According to the embodiment, the concentration of the additional dopant in the back
surface field layer is higher than the concentration of the additional dopant in the
emitter layer. And thus, the contact resistance of the back surface field layer with
the second electrode
can be increased by the additional dopant of the back surface field layer, while maintaining
the other properties of the solar cell.
[0089] Also, because the mass spectrometer is not used during the ion-implantation for forming
the back surface field layer, the manufacturing cost can be effectively reduced.